If the valley slopes at 17 deg, what's the pitch of the common rafters?

In South Africa, the minimum roof slope for using concrete tile without waterproof underlayment is 17 degrees. What would the pitch/slope of the common rafters be if the valley sloped at 17 degrees?

I know there’s somebody out there better at math than I am!

My estimate was 4/12. Looks like I was pretty close.

**Roof Pitch to Degrees **Equivalents This drawing demonstrates how many degrees rise for each pitch of a typical roof. Look at the column labeled pitch,
then look under degrees to get the corresponding amount of degrees. Example; 8/12 pitch = 33.75 degrees.

        To see Roof pitch Visual Examples go [here](http://roofgenius.com/Roof-Pitch-Examples.asp)How to determine pitch ? Go [here](http://roofgenius.com/roofpitch.htm)

Chris, you’re up late! It’s about 11AM here.

Thanks for responding, but at 4:12, which is a little over 18 deg., the valley would be much less, probably 14 degrees. or so.

I need to know the pitch of the common rafters when the valley is 17 degrees.
So that puts the valley at just under 4:12, I’m guessing the pitch of the common rafters is going to be around 6:12.

I’m not sure how to calculate it and can’t find it online.

i’ll assume no one has a square where you’re at
and tools are much easier than the bluebook calculations

they have now have apps
i haven’t tried, had the need for it and don’t know if it would work in your locale


Thanks Barry, but I don’t know what to enter. If I could get my hands on a speed square I’d be able to tell right away.

Maybe it’s too early in the morning, but I’m not exactly sure what your asking. 17 degrees is 17 degrees. It matters not what the covering is. The covering and the valley follow the same rafters so it is still 17 degrees. See this from Chris’ link… http://roofgenius.com/roofpitch.htm
Am I missing something here?

do you need a printed image

Kent knows the valley is 17 degrees just over 5:12 slope
Common rafter would be just under a 4:12 slope
see prior image and call if i need to peel you off the rafters :mrgreen:


That speed square in the pic is an antique! We use “New Math” now! :mrgreen:

<<<Off to Starbucks to get my eye’s adjusted>>>

I’ll try again;

A true valley slopes at 17 degrees. If you put a roof gauge in the valley, aligned with the valley rafter, it reads 17 degrees.

On that roof, what’s the pitch of the common rafters?

An alternative:

If anyone has a speed square, lay it on the edge of a board so that the hip scale reads 4&12. Now, without moving anything, what does the common rafter scale read? Reading in degrees?

I found the common rafter would be 23.33 degrees.
I made a drawing on Sketchup (free edition and see attachment).
I used the metric system.
Made the base line at 10m, then in order to get the 17 degrees angle, I calculated that the opposite side of the roof pitch needed to be Tan 17x10= 3.05m
Now, I have my triangle.
From there, I did a simple squared roof on each side of the valley, then measured all sides of the new triangle in order to calculate the common rafter pitch by doing:
Tan-1 (3.05/7.071)= 23.33 degrees.
Unfortunately, the free edition won’t give me the angle, so I calculated it with the dimensions on the drawing (anybody else can proof this)

Muchas gracias, Will. You lost me, but math is my weak point. What you came up with sounds about right.

As a funny aside…

Nancy, my Zulu maid, was here today and she had the TV on, which I don’t usually watch, because a lot of it’s in Zulu and because the shows are real bad. There was a game show on, and it was an algebra game show hosted by a hot black babe in a wheelchair. There were 2 teams of 2 high school kids each, one team black kids, one team white.

The babe would asked these algebra questions that I couldn’t understand, they’d cutaway to a real thin black guy in his early 30’s for further explanation and then cut to a commercial (very different commercials here, women scrubbing laundry by hand in tubs in the yard to sell detergent).

When they came back, a white guy in his early 40’s would explain the problem and solve it, but when he explained it, he would make things rhyme and include algebraic terms like “smileyface” and “frownyface” to describe parts of the equation.

Then they’d cut back to see which team got it right.

African TV in Kwazulu-Natal is pretty different from TV in the US!

I use an android app on my phone for determining pitch or angel.


There you have it: :Dx:D/:(=:mrgreen:


The angle of the common rafter = Tan-1(Tan 17°/Cos 45°) = 23.38°

That translates to a 5.19:12 pitch.

And that’s the faster way of doing things :smiley:
Thanks for the formula!

No, the faster way is to have one of these;






Thanks Randy!

Now that you have a formula, can you guys with the construction calculators figure out the common pitch if the valley is 12.5 degrees?

17.41 degrees

You’re getting pretty fast at that!